I very much doubt that it will be the air cylinder that is leaking. If you really think so, just take it off, fill it up, set aside for a few days and see if the pressure gauge has gone down.
The HW100 is probably the easiest PCP to work on. Anyone with some hex keys can take it to bits in minutes, re-seal, and put it back together. It is a most excellent, user friendly, design.
You can get a kit of everything needed from e.g. Bagnall&Kirkwood. Seven quid. A great retailer and a local shop, to me, at times.
This listing is for a full extended seal kit for the HW100/HW101 (Also known Beeman HW100). Fits all versions of the HW100/HW101 including BP/Bullpup. We include all seals for both .22 (5.5mm) and .177 (4.5mm) just do not use the one you do not require. This kit includes ALL internal and...
spares.bagnallandkirkwood.co.uk
You will also need some suitable o-ring lube. Which might cost as much again.
There might be vids online showing how to do it. I did take mine to bits, out of curiosity. Re-lubed. Then put it back together again as-was. Half an hours work, and I was learning as I went along.
My HW100 has been going strong for probably a decade with no need to mess. With PCPs, they benefit from regular use, to keep the seals exercised. Congratulations, you have one of the best PCPs ever made.
If you can't DIY, there may be some local shop, or person, who can.